If Cena's still holding the WWE Championship, then I hope he drops it to someone at Summerslam. That would be the perfect place to put over a new young guy.
If Cena's not holding the WWE Championship (unlikely, but I'm just thinking of the scenarios), then I can see a big main event involving The Shield vs Cena and 2 other guys (again) in some gimmick match.
CM Punk's apparently returning around Summerslam. You have to think that he'll be involved in something big around or even at the event if that happens.

They're talking about doing Taker vs Lesnar at Summerslam at the moment. It's a main event match, but likely won't be THE main event, since that has to involve Cena.

Cena (c) vs Lesnar
Punk (c) vs HHH
Either match could be for the title
Punk wins mitb.Raw after Lesnar beats up Cena so Punk can cash in.HHH says he will take the title off Punk and Cena wants Lesnar.
OR
Punk starts a fued with the Mcmahon family again which leads to him facing HHH and he also win MITB.
Lesnar retuns to and says he wants revenge on Cena and it just so happens that cena is champion.Lesnar beats Cena and next night on Raw Punk comes out to cash in on Lesnar,Punk hits the fingerpoke of doom on Lesnar and pins him for the title leading to Heyman,Punk & Lesnar finally being in the same angle.But then Lesnar gets fired to write him off TV.Heyman can explain this by saying that Lesnar just wants to beat people up and doesnt care about titles where as Punk wants to be the best.

If we have Undertaker I would do
Undertaker vs Punk (rematch from mania which Punk wins)
Cena (c) vs Lesnar (with the fingerpoke of doom next night so Punk cashes in)

If we have both Rock and Taker I would do
Cena (c) vs Rock vs Punk wwe title (Punk cashes in mitb to make triple threat and wins)
Undertaker vs Lesnar(Taker returns to confront shield,Heyman is behind the shield so Lesnar gets involved.